This invention relates to control of stored magnetic energy in power converter transformers.
In one class of power converters, called single-ended switching power converters (10, FIG. 1), a primary switching element 20 is used to repetitively connect an input source to the primary winding 329 of a power transformer 25 (FIG. 1) during a portion of each of a series of converter operating cycles. In some such converters, called xe2x80x9csingle-ended forward converters,xe2x80x9d forward energy transfer from the input source toward the load occurs during the time that the switch is closed. In other topologies, called xe2x80x9csingle-ended flyback convertersxe2x80x9d, energy is stored in the transformer during the time that the switch is closed and is transferred forward to the load after the switch is opened. In yet other single-ended converters a portion of the energy may be transferred during both the on and off times of the switch. In general, practical converters of this kind must incorporate circuitry for managing the effects of magnetic energy storage in the transformer. For example, in converters in which energy is transferred forward during the on-time of the switch, circuitry is typically included to deal with the magnetizing energy which is stored in the transformer at the time that the switch is opened (e.g., to provide means for xe2x80x9cresettingxe2x80x9d the transformer); more generally, the effects of magnetic energy storage in the leakage inductance of the transformer must also be managed, e.g., to prevent the energy stored in the leakage field from overstressing the switch when the switch is turned off.
A wide variety of reset circuits (also called core reset circuits because almost all switching power converter transformers include a magnetic core), for use in single-ended forward converters, are described in the literature. These circuits differ in terms of their efficiency, cost and impact on converter power density. One such circuit 15, shown in FIG. 2, is described in Clemente, et al., xe2x80x9cA Universal 100 KHz Power Supply Using a Single HEXFET,xe2x80x9d International Rectifier Corporation Applications Note AN-939, December 1980. One of the advantages of this circuit is that it can maintain the minimal value of peak switch voltage consistent with converter input voltage and duty cycle; one of its disadvantages is that it is dissipative and therefore compromises both converter efficiency and power density. Another circuit 30, of the kind described in Vinciarelli, xe2x80x9cOptimal Resetting of the Transformer""s Core in Single Ended Forward Converters,xe2x80x9d U.S. Pat. No. 4,441,146, (incorporated by reference) is shown in FIG. 3. This circuit has several advantages, among which are the following: (1) it, too, minimizes the peak voltage seen by the switch, (2) it provides for bipolar transformer 25 core excitation, thereby maximizing utilization of the available flux swing in the transformer""s core and allowing for a smaller transformer, and (3) it is essentially lossless.
When used in single-ended forward converters, core reset circuits 15, 30 of the kinds shown in FIGS. 2 and 3 will also inherently control the effects of leakage energy by providing a capacitive sink which xe2x80x9csnubsxe2x80x9d (i.e., clamps) the effect of the release of the leakage energy on switch voltage. In other kinds of converters, single-ended or otherwise, circuits 15, 30 of the kinds shown in FIGS. 2 and 3 may be used solely as snubber circuits to manage and control the effects of stored leakage energy. For example, in flyback converters all of the magnetizing energy stored in the transformer 25 during the time that the primary switch 20 is closed is magnetically coupled into the transformer secondary winding and released to the load after the switch opens. While this provides a natural mechanism for reducing the magnetizing energy to zero it does not provide a mechanism for managing transformer 25 leakage energy, which can cause excessive switch 20 voltage when the switch 20 is opened. Circuits 15, 30 of the kinds shown in FIGS. 2 and 3 (the simplified converter schematics in the Figures can be modified to correspond to a single-ended flyback topology by simply reversing the polarity of one of the transformer 25 windings) can be used to clamp the voltage which might otherwise be caused by the stored leakage energy. For example, in Ogden, xe2x80x9cImproved High Frequency Switching in Coupled Inductor Power Supplies,xe2x80x9d European Patent Application 0 350 297, a circuit essentially identical to that of the circuit 30 of FIG. 2 is used to both clamp the leakage inductance energy in a flyback converter and reroute it forward toward the load.
In an aspect, the invention features apparatus (and a method) useful in a switching power converter having a transformer and a primary switch for connecting a DC input source to a primary winding of the transformer during a portion of each of a succession of converter operating cycles, the apparatus includes a reset capacitor and a reset switch. Reset circuitry cooperates with the reset switch to connect and disconnect the reset capacitor in a manner which provides for resetting the transformer and which allows a current having a non-zero average value to flow in the reset switch.
Embodiments may include one or more of the following features. The reset circuitry may open and close the reset switch. The reset circuitry may cause the connecting and disconnecting to occur at times based on switching of the main switch. The reset switch may be connected in series with the capacitor. The reset circuitry may include circuit elements which inhibit bidirectional energy flow between the capacitor and the transformer. The apparatus may only allow transfer of magnetizing energy between the reset capacitor and the transformer if the polarity of the voltage across the reset capacitor is of a particular polarity. The apparatus may be connected in parallel with a winding on the transformer. The winding may be the primary winding, a secondary winding, or an auxiliary winding. The reset switch may be a unidirectional switch (e.g., a MOSFET) in parallel with a unidirectional conducting element (e.g., the intrinsic body diode of the MOSFET), the unidirectional switch and the unidirectional conducting element being arranged to conduct in opposite directions. The unidirectional switch may be a MOSFET in series with a series diode, the series diode and the MOSFET being poled to conduct in the same direction. The circuit elements may include a reset diode connected in parallel with the capacitor. Bidirectional energy flow may be allowed only if the voltage across the reset capacitor is of a particular polarity. The particular polarity may be that which will result in a reversal in the polarity of transformer magnetizing current during the time that the reset switch is closed. The reset circuitry may include circuit elements which inhibit bidirectional energy flow between the capacitor and the transformer. The circuit elements may include a reset diode connected across a series circuit comprising the reset capacitor and the series diode. The switching power converter may be a forward power converter, a zero-current switching converter, or a PWM converter. The reset circuitry may open the reset switch prior to the ON period of the primary switch, may close the reset switch during the OFF period of the primary switch, and may keep the reset switch open throughout the ON period of the primary switch.
In an aspect, the invention features a method for limiting the slew rate in a switching power converter which includes a transformer and a reset circuit of the kind which non-dissipatively recycles the magnetizing energy stored in a transformer during each of a succession of converter operating cycles. The method includes sensing the magnetizing current which is flowing in said transformer, and allowing initiation of another converter operating cycle only if the magnetizing current meets a predefined criterion for safe converter operation. In embodiments of the invention the predefined criterion may be the magnetizing current passing through a predefined value, e.g., zero. Among the advantages are the following. Resonance between the transformer and the reset capacitor is inhibited. Very low dissipation is achieved. The selection of Cr can be made based solely on the operating requirements of the converter.
Other aspects, features, and advantages will become apparent from what follows.